1. Field of the invention
This invention relates to a method and an apparatus for processing images for use in an image reading apparatus, wherein color space data of a desired colorimetric system can be stably obtained from image information carried by an original.
2. Description of the Related Art
In order to simplify work steps and improve quality of images in the production of a master film in the printing and platemaking field, for example, there is widely used an image scanning, reading and reproducing system which electrically processes image information recorded in an original.
In an image reading apparatus of the image scanning, reading and reproducing system, the original accommodated and positioned in an original cassette is transferred to a reading unit, and image information borne by the original is photoelectrically read there by CCDs or the like.
It is impossible, by the way, to accurately reproduce the image information recorded in the original without any calibration of the image reading apparatus in use, because the characteristic of an optical system in the reading unit is generally different from apparatus to apparatus.
As a calibration method to be easily thought of, there may be raised a direct mapping method. In the direct mapping method, standard data is first obtained by reading with a spectrodensitometer a chart on which standard brightness, chroma and hue are shown stepwise. Then, a conversion table is prepared for converting density data obtained by reading the chart with an image reading apparatus into corresponding standard data. Thus, the calibration of image data obtained from a desired original is enabled by the conversion table. Here, image data having the densities falling between respective steps of the chart is calibrated by interpolation, optimization, etc.
In order to obtain a highly accurate image data by the direct mapping method referred to above without relying on the interpolation or the like, it is necessary to use a chart having as many steps as possible. If dust, scratch, etc. is at even one point of the chart, an improper conversion table can be produced, which makes it impossible to obtain desired image data. It is actually impossible, on the other hand, to read all of the many steps on the chart by the spectrodensitometer with a high accuracy. As a result, an unnatural image is sometimes reproduced from the calibrated image data.